Polytetrafluoroethylene resins are employed in various fields because of their excellent chemical resistance, heat resistance and mechanical properties. For example, porous films made of the PTFE resins are widely employed as filters for corrosive substances or high temperature substances utilizing the above-mentioned excellent properties, and also employed as electrolytic membranes, fuel batteries or medical tubing such as artificial blood vessels and artificial tracheas.
Recently, porous films having pores of nearly round and uniform diameters are demanded in accordance with developments in the semiconductor art or the molecular biology art, because extremely small sized impurities can be removed by the use of those films. As such porous films, PTFE porous films have been the subject of increasing interest.
In a conventional process for preparing porous films of the PTFE resins, extremely small sized particles of PTFE (called fine powder) having a mean primary particle diameter of 0.1 to 0.4 .mu.m obtained by emulsification polymerization of tetrafluoroethylene are used, and this process comprises the steps of adding a liquid lubricant to the fine powder, compression-preforming the mixture, subjecting the obtained preform to extrusion and/or calendering to give a film, removing the liquid lubricant from the film to obtain a PTFE film, and then subjecting the PTFE film to uniaxial or biaxial stretching under heating. For example, Japanese Patent Publication No. 53(1988)-42794 describes a process for preparing a PTFE porous film characterized by heating a sintered PTFE resin film to a temperature not lower than 327.degree. C. then slowly cooling the film, heat-treating the film so that its crystallinity becomes not less than 80%, and then uniaxially stretching the film within the temperature range of 25.degree. to 260.degree. C. in a draw ratio of 1.5 to 4 times.
In the above process, however, it is difficult to prepare a PTFE porous film having nearly round pores and desired pore sizes. Further, the PTFE film used as a raw film in this process sometimes has pinholes, voids or scratches, and hence the obtained porous film is not always sufficient in mechanical strength.
For solving the above-mentioned problem, there has been proposed a process for preparing a polytetrafluoroethylene porous film which comprising the steps of compression-molding a PTFE resin powder having a mean particle diameter of 1 to 900 .mu.m obtained by a suspension polymerization to prepare a PTFE preform, sintering the preform at a temperature not lower than 327.degree. C. processing the preform into a film, then sintering the obtained film at a temperature not lower than 327.degree. C. then quenching the film at a cooling rate of 70.degree. C./hr or more to decrease crystallinity of the film to not more than 55%, and uniaxially or biaxially stretching the film in a draw ratio of 1.3 to 6.5 times under heating at a temperature of 100.degree. to 320.degree. C.
However, the PTFE porous film prepared by this process is low both in water permeability and porosity. If the thickness of the film is made smaller to increase water permeability, the film is liable to be broken off during the stretching treatment. Further, even if the stretching treatment is possible, the resulting porous film sometimes suffers visible pinholes having a hole diameter of not less than 0.1 mm. In this process, moreover, sequential stretching can hardly be accomplished made because the porous film is liable to be broken during the stretching treatment, and therefore only single part production (sheet form production) using a simultaneous biaxial stretching is possible. Hence, it is difficult to prepare porous films with high productivity.